With the development of large offshore wind farms for producing electric energy, transmission losses occurring along transmission lines between a wind farm and a connection to a distribution grid on land may become significant.
In such cases, HVDC (high-voltage, direct current) technology has distinct advantages in terms of low losses in comparison to the common AC (alternating current) technologies. In particular, by using a high voltage (up to e.g. 800 kV), the current flowing in the transmission lines—and thereby the resistive losses—is significantly reduced when a given amount of power is transmitted. Thus, in terms of losses, HVDC is advantageous for long distance power transmission and may be considered as being ideal for offshore wind power applications. The use of fewer number of wires for transmission also means reduction in costs.
However, in order to handle the high voltage and high power in the HVDC transmission, especially under increasingly stringent performance requirements, some sophisticated power converter systems have to be applied, which can be technically challenging and often means high cost.
In known implementations, the AC output of a grid converter of a wind turbine is connected to a step-up transformer, and then converted to HVDC through a converter. Two of the common converter topologies are line-commutated current source Thyristor converters and IGBT based voltage source converters (VSCs).
Thyristor converter systems have been the dominant technique for HVDC transmission, which can use voltages as high as 800 kV. In this converter type, the firing angle is used for control of the DC voltage and the power flow. It consumes reactive power and the AC currents contain low-frequency harmonics. Thus, phase compensation is normally used to improve the power factor, and large filters are required to reduce the current distortion.
The HVDC transmission with VSC technique has the advantages of high control bandwidth and low current harmonic distortion, and thus low requirements on the line filters. This kind of system is in fast progress, and two examples are Siemens HVDC Plus and ABB HVDC Light. Both systems employ multilevel converter techniques in order to accommodate low voltage rated power components (e.g., IGBT) and relatively low switching operations, and hence the systems tend to be relatively more technically challenging and more expensive.
There may be a need for a simple and cost-effective way of connecting an electrical power generator to an HVDC transmission system.